The present invention relates to a motor control apparatus, more particularly to an improved motor control apparatus having a thyristor chopper.
In recent years, circuits to control electric power supplied to electric motors have been proposed, in which the voltage is applied to the motor intermittently at a predetermined period through a chopper circuit using thyristors or power transistors so that the average value thereof is maintained at a desired value. In such a control circuit, it is desired, in addition to reduction of size, that a duty factor of the chopper circuit, i.e., the ratio of conductive time to the sum of conductive and non-conductive times be controlled over a wide range.
In U.S. patent application filed on Aug. 25, 1978, of "A MOTOR CONTROL APPARATUS WITH AN IMPROVED THYRISTOR CHOPPER CIRCUIT", invented by H. NARITA et al and assigned to Hitachi, Ltd., now U.S. Pat. No. 4,209,733, issued June 24, 1980 there is shown a motor control apparatus to supply current from a D-C source to an electric motor through a thyristor chopper circuit. The thyristor chopper circuit is provided between the terminals of the D-C source in series with an electric motor, and has main and auxiliary thyristors. A series connection of a capacitor and a reactor, forming commutating elements, is connected across the auxiliary thyristor and oscilates to generate a pulse of current through a forward diode in order to turn off the main thyristor when the auxiliary thyristor is turned on. In addition, a saturable current transformer is so provided that through a primary winding thereof chopper current flows, and the secondary winding operates as the reactance of the commutating elements. Current flowing through the secondary winding therefore depends on the chopper current flowing through the primary winding.
Other relevant prior art is as follows:
(1) R. E. Morgan, "Time Ratio Control with Combined SCR and SR Commutation", IEEE Trans. on Communications and Electronics, Vol. 83, pages 366-371, July, 1964. In this article, there is shown a commutating circuit for TRC that combines an auxiliary SCR and a SR. This commutating circuit contributes to the reduction of the cost and size of the auxiliary SCR. However, it has drawbacks in that two kind of reactors L.sub.C and SR.sub.C are necessary, the former having a relatively high inductance and the latter being a saturable reactor, and in that commutating current through the auxiliary SCR is always determined by commutating elements of a capacitor C.sub.C and the reactor L.sub.C.
(2) W. McMurry, "Silicon-Controlled Rectifier D-C to D-C Power Power Converters", IEEE Trans. on Communication and Electronics, Vol. 83, pp. 198-203, March, 1964. This shows various ways in which saturable transformers are used in silicon-controlled rectifiers.
(3) U.S. Pat. No. 3,875,486, "Motor Speed Control Circuit". This shows Jone's chopper circuit having a transformer. This Jone's chopper, however, differs from an ordinary chopper circuit in construction and operation.
(4) U.S. Pat. No. 3,903,465, "Chopper Control System". In this chopper control system, the application of the chopper-off signel to the chopper is prohibited by a prohibit circuit.